It is known in the automotive industry to provide for wireless monitoring of vehicle tire parameters, particularly tire pressure. In such tire pressure monitoring systems, tire pressure sensors and radio frequency (RF) transmitters are mounted inside each tire, typically adjacent the inflation valve stem. In one prior art system implementation, the tire pressure sensed by the tire pressure sensor is transmitted by the transmitter to a central receiver/controller located on-board the vehicle. The tire pressure information delivered to the receiver/controller by the RF signals from the transmitters is subsequently conveyed to a vehicle operator or occupant, typically in the form of a display.
To recognize the particular tire location associated with an RF signal received from a tire transmitter (e.g., front left (FL), front right (FR), rear left (RL), rear right (RR)), such tire pressure monitoring systems are programmed in an initialization or sign-up operation. That is, in order to provide a vehicle operator with information specific to each vehicle tire, programming of the tire pressure monitoring system is undertaken, typically by a technician, so that each RF signal from a tire transmitter will be associated with a particular tire location. In the above manner, if a “low tire pressure” indicator is provided to an occupant of the vehicle, the indication will also include an indication of which tire has such condition.
In one prior art example, the tire pressure monitoring system uses a magnetic reed switch in each tire for such programming. More particularly, after the on-board vehicle/controller is placed into a program, initialization, or “learn” mode, the magnetic reed switch in each tire is activated by a technician using a magnet. Such activation causes the tire transmitter in the tire to transmit a tire pressure signal to the controller on the vehicle. In that regard, each pressure sensor and/or transmitter has a unique identification code associated therewith, which identification code is transmitted with the tire pressure signal. Using such identification codes the controller associates each received tire pressure signal with a particular tire location.
Such operation, however, can create problems when tires are subsequently rotated or changed from their initial locations to new locations, or a vehicle tire is replaced (e.g., a tire replacement or use of the spare tire). Each time the vehicle tires are rotated or a tire is replaced, the manual initialization procedure must be repeated to ensure that the system continues to operate properly by conveying accurate information, including tire location, to the vehicle operator. This initialization requirement makes tire rotation (or other tire changes) more complex, and increases the possibility of inaccurate operation of the system.
The tire transmitters used in such tire pressure monitoring systems are typically battery powered. As a result, a tire transmitter has a limited amount of functioning time before its battery must be replaced. To help conserve battery power, the transmitters typically transmit tire pressure information at short, predetermined time intervals (as opposed to continuously) when the vehicle is moving. In addition, once the vehicle has been stationary for a predetermined amount of time, the transmitters may transmit tire pressure information at longer predetermined time intervals.
Conventional tire monitor modules employ an accelerometer to increase the periodic rate at which transmissions are made to vehicle controller/receiver. Consequently, when the vehicle is at rest, a rate at which tire data is transmitter may be at a first, relatively long time interval, such as 30 minutes. Alternatively, as the accelerometer senses the vehicle increasing in speed above a threshold value, the tire transmitter may transmit tire data at a second, relatively short time interval, such as about 1 minute. Typically, the accelerometer is a mechanical or micro-electromechanical (MEM) device that can be prone to fail. Further, some accelerometers have difficulty meeting the stringent system tolerances. Lastly, the accelerometer adds to the complete solution cost.
In any event, each prior art tire transmitter operates independently of the other transmitters. Consequently, when two or more tire transmitters associated with a vehicle transmit tire pressure signals or data simultaneously, a data collision can result at the vehicle central receiver/controller, which can adversely affect proper operation of the tire pressure monitoring system. One prior art solution to the above collision problem involves implementation of anti-collision algorithms, wherein each tire transmitter transmits the tire data in a redundant manner that is spaced apart from each other in time (e.g., eight transmissions). In addition, each tire transmitter spaces apart the redundant data with a different time interval. Consequently, when data is sent, although data collisions are not necessarily avoided, the redundant data transmissions ensure that at least some of the data finally gets through to the vehicle receiver/controller. This prior art solution is effective, however, since more time is employed to transmit the data, more battery current is employed, thereby resulting in a shorter battery life of the tire sensor module. In addition, the redundant transmissions increase the average radiated emissions that are measured by governmental regulators in selected regions, and thus in some instances are undesirable. Lastly, since the vehicle controller/receiver is required to stay active for a longer period of time, more current is required of the vehicle batter, which in some instances may be undesirable.
Another prior art solution employed to communicate tire pressure data from each sensor to the vehicle controller/receiver employs low frequency (LF) initiators that are local to each tire pressure sensing module at each respective tire. Each LF initiator module includes an antenna that transmits a low frequency initiation signal to the tire monitor module, thereby “awakening” the module for transmission of tire data therefrom. Because the low frequency signal (e.g., about 125 KHz) power decays extremely quickly, only the tire module local to the respective LF antenna is activated. By having each LF initiator module operating at distinct, different timing intervals, data collisions are avoided.
The prior art LF initiator solution, however, may require additional certification from various regulator commissions due to the LF transmissions, and requires LF antennas to be located at each wheel location. Further, the LF initiator requires some form of module and wiring to each antenna location. In addition, to generate and transmit the LF initiation signal, a high current and/or high voltage driver is required to generate sufficient field strength for the LF receiver at the tire sensor. Lastly, each tire sensor module requires continuous current from its local battery to activate the LF receiver in the module.